1. Field of the Invention
This invention relates to a method and apparatus for threading a helix onto a radial bristle cosmetic brush. More particularly, the invention relates to a method and machine for threading a helix on a twisted wire core cosmetic brush such as a mascara brush.
2. Description of the Prior Art
Brushes 27 having bristles extending radially from a central core are well known in the art (see, for example, FIG. 1). They may be made by fixing tufts of radially extending bristles in bores in a molded plastic central core. Alternatively, the core is commonly of the twisted wire type. A twisted wire core brush is typically comprised of a core 10 formed from a single metallic wire folded in a generally unshaped configuration to provide a pair of parallel wire segments. Bristles 18, usually comprised of strands or filaments of nylon or similar material, are disposed between a portion of a length of the wire segments. The wire segments are then twisted about each other, forming a twisted wire core 10 (also known as a helical core), with the bristles 18 secured in the twisted wire core substantially at their midpoints so as to clamp them. The bristle tip distribution generally approximates the helical or spiral path of the twisted wire core. In this way, a bristle portion or bristle head 28 is formed with regularly disposed radially extending bristles secured in the twisted wire core in a helical or spiral arrangement. The twisted wire core generally has a handle end 8 that extends beyond the bristle portion 28, thus providing a portion of the core for mounting the brush to a stem or rod 24, which in turn may be attached to a cap 26 that also serves as a handle. The cap forms the closure for a cosmetic container 30, such as, for example, a mascara tube. Reciprocal threads 16 and 29 may be provided on the cap and container, respectively, to secure the closure. The brush, rod and cap together form a brush assembly 22 (also known as an applicator). A longitudinal axis 23 is defined by the core 10. See, for example, U.S. Pat. No. 4,887,622 to Gueret, U.S. Pat. No. 4,733,425 to Hartel et al. and U.S. Pat. No. 5,370,141 to Gueret.
Also known is a radial bristle brush 27 as described above with a helix 9 (also referred to herein as a ‘helical coil’ or an ‘added helix’) mounted or threaded about the core and within the bristles 18 in the bristle head 28 (see FIG. 3). The helix 9 is made of metal, plastic or other suitable material, and is added to the brush to redistribute the bristles 18 of the underlying brush 27. A brush of this type is disclosed in U.S. Pat. No. 6,295,994 to Thayer et al., incorporated herein by reference in its entirety. The helix 9 (see FIGS. 2–3) has a generally cylindrical body comprised of a spiral series of successive loops or coils 7 winding about a longitudinal axis 17 defined approximately through the center of the longitudinal body. The cylindrically configured helix 9 is positioned coaxially about the core 10 of the brush 27 such that at least some bristles 18 extend radially between the successive loops or coils 7 of the helix. The characteristics of the helix, such as, for example, the internal dimension, the pitch, the thickness of the helix body, etc., are each selected to, for example, change the initial orientation of at least some of the bristles when the helix is in position about the core. Thus, the helix changes the physical arrangement of the brush by reorienting at least some bristles to yield a brush with selected characteristics. On a brush without a twisted wire core, the added helix can simulate the bristle pattern of a twisted wire core brush by providing a spiral arrangement of bristles. On a twisted wire core brush, bristles may be re-arranged to yield any one of a variety of bristle distributions.
However, the automated assembly of a helix onto a radial bristle cosmetic brush presents some challenges. For example, the assembly requires care to avoid undesired bristle re-distribution or damage. Forcing a helix onto the brush with too much pressure, or at a feed rate that is incompatible with the pitch of the helix or the bristle arrangement of the underlying brush may flatten or damage bristles, or undesirably bias bristles towards the brush tip or handle. Also, cosmetic brush assemblies such as mascara brushes come in a variety of shapes and sizes, with stems and/or caps of various dimensions. Because the helix may be mounted on the brush after the brush is secured to a stem and/or cap, the automated assembly apparatus must be readily adaptable to accommodate a variety of brush, stem and cap dimensions and arrangements. Accordingly, there is a need for an apparatus that can conduct the automated assembly of a helix onto a radial bristle brush in a manner that overcomes the foregoing challenges.